As is known in the art, Indium Nitride (InN), Gallium Nitride (GaN), Aluminum Nitride (AlN), Boron Nitride (BN) and all of their associated alloys including Inx(AlyGa1-y)1-xN (where 0≤x≤1 and 0≤y≤1) and Bz(Inx(AlyGa1-y)1-x)1-zN (where 0≤x≤1 and 0≤y≤1 and 0≤z≤1) are a family of materials known as Group III-Nitrides. The Group III-Nitrides are used in power and microwave transistor device electronics in part because of their breakdown voltages, bandgap energies, and saturation velocities. One of the principal issues related to device performance is the crystal quality of the epitaxial material grown from Group III-Nitride materials as most epitaxial Group III-Nitride hetero-structures are grown on lattice mismatched substrates due to the high cost and low availability of large diameter bulk Group III-Nitride substrates. The lattice mismatch results in the formation of numerous crystallographic dislocations detrimental to device performance. Typically Group III-Nitride buffer layers are grown in excess of one micron in thickness before the active device regions are grown to allow the material to relax and to reduce as many dislocations as possible through the growth process while creating a flat surface morphology.
As is also known in the art, during the growth of GaN by plasma assisted molecular beam epitaxy (PAMBE), gallium atoms are evaporated onto a substrate surface in the presence of an activated nitrogen flux provided by a plasma source. The surface morphology of GaN during growth depends on the substrate temperature and the ratio of the gallium flux to the plasma activated nitrogen flux and has been commonly separated into three principal regimes; an N-stable; an intermediate; and a Ga-droplet growth regime. The N-stable growth regime occurs when the net gallium flux (incident gallium flux minus the rate of desorption from the surface) is less than the flux of activated nitrogen supplied to the surface from the plasma source. The Ga-droplet growth regime occurs when the net gallium flux exceeds the activated nitrogen flux and leads to the accumulation of a gallium wetting layer greater than 2.5 monolayers in thickness and to the formation of gallium droplets on the growth surface. The intermediate growth regime occurs when the net gallium flux exceeds the activated nitrogen flux, but the desorption rate of the gallium is sufficiently high to prevent gallium droplets from accumulating on the growth surface. The intermediate growth regime covers all growth between the N-stable and the Ga-droplet regime and includes various coverages and thicknesses of the gallium wetting layer up to 2.5 monolayers in thickness.
The N-stable and the intermediate growth regimes are also classified as three dimensional (3D) growth regimes as both produce pitted surface with a rough surface morphology. The N-stable GaN growth regime typically produces a rough faceted and heavily pitted surface morphology with a tilted columnar structure and a high density of stacking faults. The surface morphology in the intermediate GaN growth regime varies significantly depending on the amount of gallium accumulation on the growing surface with a continuous reduction in surface pit density and growth planarization as the amount of gallium increases. There is also a transition from layer-by-layer growth to step flow growth within the intermediate growth regime as the gallium flux increases and stabilizes the presence of a 1 to 2.5 monolayer thick gallium adlayer on the surface.
The Ga-droplet GaN growth regime produces atomically flat surfaces and is also classified as a two dimensional (2D) growth regime. If the accumulation of gallium on the surface is allowed to continuously increase during the growth process, the liquid gallium eventually will begin to randomly degrade the quality of the GaN surface. Growth at the crossover boundary, between the Ga-droplet and intermediate growth regimes, has been reported to result in atomically flat surfaces dominated by step flow growth. However, temperature fluctuations of less than a few degrees across a wafer can cause the growth to deviate from the crossover boundary and lead to significant morphological variations across the wafer. To mitigate this problem a number of “modulated” growth methods were developed having short periods of GaN growth in the Ga-droplet GaN growth regime followed by short periods where excess gallium is removed from the surface. In one example of a modulate MBE growth method, the gallium and nitrogen shutters are opened simultaneously for a period of time with the gallium flux high enough for growth to occur in the Ga-droplet regime and then both shutters are simultaneously closed for a period time to allow excess gallium to desorb from the growth surface. In this example, the substrate temperature must be hot enough to desorb the excess gallium from the surface. In another example of a modulated MBE growth method, multiple gallium sources are used for growth with the shutter to one source continuously open and the shutter to the second source alternating between opened and closed. When the second shutter is open, growth occurs in a Ga-droplet regime and excess gallium accumulates on the surface. When the second shutter is closed, the excess gallium begins to be consumed by the growth process and the shutter is kept closed until the excess gallium is cleared from the growth surface. When the excess gallium is cleared, the second shutter is reopened to resume growth in the Ga-droplet regime. A modulation period commonly occurs over a couple hundred Angstroms of growth.
More particularly, during the hetero-epitaxial growth of GaN on a lattice mismatched crystalline substrate, a high density of crystallographic dislocations are produced near the interface of the growing epitaxial GaN film to reduce the strain resulting from the epitaxial misfit. High crystallographic dislocation densities in GaN-based high electron mobility transistors (HEMTs) are undesirable as they lower the electron mobility and raise the sheet resistivity within the two-dimensional electron gas (2 DEG) of the device. Although the modulation growth methods for GaN produce smooth surface morphologies throughout the growth, they also enable the crystallographic dislocations in the GaN to propagate parallel to the growth direction, which keeps dislocation interactions low and dislocation densities high.
According to a paper by C. D. Lee et al., “Role of Ga flux in dislocation reduction in GaN films grown on SiC (0001)”, Appl. Phys. Lett. 79, 3428 (2001), the probability of crystallographic dislocation annihilation (A) and/or combination, where a pair of dislocations in the GaN film combine (C) or become annihilated (A), is greatly enhanced when dislocations are clustered rather than randomly spaced since the interactive force is, in general, inversely proportional to the distance between dislocations. When GaN is grown under more nitrogen rich conditions, the surface roughens as the mobility of adatoms on the surface is reduced. The crystallographic dislocations in these films are generally observed to reside in topological valleys along the GaN surface and at the lowest points of the growth surface. It has been theorized by Lee that the rough surface leads to clustering of dislocations in the topological valleys and that the clustering of dislocations leads to a higher rate of dislocation annihilation or combination in the GaN film.
It was later shown by Waltereit et al., “Structural Properties of GaN Buffer Layers on 4H—SiC(0001) Grown by Plasma-Assisted Molecular Beam Epitaxy for High Electron Mobility Transistors”, Jap. J. Appl. Phys., 43, L1520 (2004) that the advantage of crystallographic dislocation reduction due to the roughening of the growth surface is limited to the first 100 nm of GaN growth. Eventually all of the crystallographic dislocations (referred to in the paper as threading dislocations) become located at topological valleys and additional growth under the same growth conditions can only create additional clusters by creating significantly larger topological valleys such that two or more smaller valleys merged into a larger valley. Consequently, growth of thicker buffers in this intermediate growth regime only adds to the roughness of the surface without significantly reducing the crystallographic dislocation density. The requirement to drastically increase surface roughness to further reduce crystallographic dislocations is not compatible with device fabrication.
Some GaN layers in the prior art have been reported to be grown in two growth steps as first demonstrated by Manfra et al., in a paper entitled “Dislocation and morphology control during molecular-beam epitaxy of AlGaN/GaN hetero-structures directly on sapphire substrates”, Appl. Phys. Lett., 81, 1456 (2002). During the first step the GaN is grown under intermediate, or 3D, growth regime conditions that promote a rough surface morphology to reduce crystallographic dislocations. Manfra et al. point out that, in referring to FIG. 1 of the paper, “The 3D growth mode appears to increase dislocation interactions, and thus, reduces the number of dislocations which propagate to the surface”. Manfra et al. also point out that FIG. 1 shows that significant bending of threading dislocations (bending away from the growth direction and move laterally to some degree) is observed throughout the first 750-1000 nm of growth and that many of these dislocations intersect other dislocations “where they can annihilate if the dislocations have Burgers vectors with opposite sign. The arrows in FIG. 1 indicate a few of the closed dislocation loops seen in this film”. Manfra et al. point out that “While growth under nitrogen stable conditions appears to enhance threading dislocation interactions, the roughened surface morphology produced by continued growth under these conditions is not optimal for device structures”. Thus, in order to produce a surface smooth enough for device fabrication, a “second phase of growth of the insulating GaN buffer, the Ga flux is increased to produce metal stable growth” and “The remainder of the epilayer is grown under these Ga-stable conditions. The Ga flux is limited such that no metal droplets are observed on the surface”. Manfra et al. note that “a fairly abrupt change is observed in the defect structure seen in FIG. 1” and that “From this point, many dislocation loops have closed and the remaining threading defects are seen to propagate straight up to the film surface. The rough/smooth transition appears to minimize dislocation bending”.
For example, referring to FIG. 1A, a structure is shown having a silicon carbide (SiC) substrate with an AlN nucleation layer viewed parallel to the <1100> axis and normal to the <0001> and <1120> axes. A GaN layer is then grown epitaxially using 3D growth conditions along the <0001> axis; here using Molecular Beam Epitaxy (MBE) at a predetermined growth temperature and at a predetermined gallium to nitrogen flux ratio that results in a steady increase in surface roughness. Due to the lattice mismatch between the GaN and the underlying substrate, crystallographic dislocations (D) and significant bending of threading dislocations (B) are formed as shown in FIG. 1A. It is noted that after an initial annihilation region, these dislocations (D) continue to project along the <0001> axis. The majority of the bending dislocation interactions resulting in annihilation (A) and/or combination (C) of the dislocations and are shown to be confined within or near the 3D growth layer (an initial annihilation region). As noted by Manfra et al., with regard to the 3D to 2D (3D/2D) interface; shown in FIG. 1A, Manfra et al., states “The rough/smooth transition appears to minimize dislocation bending”.
FIG. 1B is a cross-sectional scanning transmission electron microscopy (STEM) image along the <1100> zone axis of a GaN <0001> layer structure deposited using a two-step (3D to 2D) growth method whereby the GaN near the substrate is grown at a growth temperature and gallium to nitrogen flux ratio that results in 3D growth and the remainder of the GaN layer is grown at a growth temperature and gallium to nitrogen flux ratio that results in 2D growth.